Process for treating ferrous metals



United States Patent 3,011,906 PROCESS FOR TREATING FERROUS METALS Shipp C. Davis, Chattanooga, Tenn., and William G. Ray, Aflanta, Ga., assignors to Chas. Pfizer & Co., Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Aug. 24, 1960, Ser. No. 51,546 5 Claims. (Cl. 117-50) This application is concerned with a new and improved process for porcelainizing metals. More particularly, it is concerned with an improved process for pretreating iron and steel, hereinafter referred to as ferrous metals, to prepare the surface thereof for coating with porcelain enamel by a one-coat process.

Patent applications Nos. 768,045 and 831,935, filed October 20, 1958, and August 6, 1959, respectively, both now abandoned, and patent application Serial No. 924, filed January 7, 1960, now Patent No. 2,981,634, describe and claim a novel one-coat process for porcelainizing ferrous metals comprising the steps of cleaning the ferrous metal, pickling with an aqueous solution of glycolic acid containing from about 1% to about 20% by weight of acid at a temperature of from about 160 to about 212 F. for from about 5 to about 30 minutes, depositing nickel on the surface in an amount of from about 0.12 to about 0.36 gram per square foot, treating the object with nickel deposited on the surface thereof with porcelain enamel, drying at from about 250 to about 400 F. and baking same at from about 1350 to about 1450 F. for from about 2 to 6 minutes.

It has now been discovered that provided certain conditions are met, it is possible to decrease the time that the article to be porcelainized must spend in contact with the pickling solution. This is an extremely important economic consideration in the porcelaiuizing industry since it makes it possible to enamel cover more area per unit time. These conditions include the use of critical concentrations of alkali metal thiosulfate or alkali metal sulfite, preferably sodium salts.

. A simplified flow diagram of the process is as follows:

Ferrous Metal 1 Cleaned I Rinsed Pickled, Aqueous Glycolic Acid Containing Alkali Metal Thiosullate or Sulfite Porcela' -Coated I Dried I It has been found possible in many instances to decrease the pickle time as much as 50% by the addition of one of these reagents to the glycolic acid bath. Thus, highly purified low carbon steels, i.e., those having less than 0.003% carbon, usually require approximately 20 minutes of pickle time when glycolic acid alone is used, but by the addition of a reagent of this invention, the pickle time may be decreased to as low as 10 minutes while still obtaining comparable results. With conventional enamel stock or cold rolled steel, the pickle time, when glycolic acid alone is used, is approximately 5 to 10 minutes. When using the improved process of this invention, the time can be decreased to 3 to 6 minutes.

A number of cleaning methods are available for cleaning the surface to be coated when the process of this invention is to be employed. These include alkali rinses, vapor degreasers and emulsion-type cleaners. Their purpose is to remove greases, oils, soap, dirt and drawing compounds such as soaps, borax and waxes which are applied to the surface of the metal to provide lubrication during the fabrication process. Typical alkali rinses may contain sodium hydroxide, sodium carbonate, sodium phosphate, sodium silicate or combinations of these. The part to be cleaned is simply immersed in the cleaning compound at about room temperature for a sufficient time to accomplish the desired result. usually requires from about 10 to about 20 minutes. Vapor degreasers are especially useful for removing hydrocarbon residues from the surface of the metal and in the process, the vapors of a low boiling solvent, for example, a chlorinated hydrocarbon solvent, such as ethylene dichloride, are allowed to condense on the surface of the metal to wash off the residual hydrocarbons. An emulsion-type cleaner may contain a non-ionic surfactant dissolved in naphtha or kerosene and the part to be cleaned is simply rinsed in the solution. Treatment with an emulsion-type cleaner is almost always followedby the alkali rinse described above. i

After cleaning, the article is subjected to two aqueous rinses, the first a hot rinse carried out at from about to about F. and the second carried out at from about 55 to about 65 F.

In the next step of the one-coat process, the metal to be porcelainized is pickled with aqueous glycolic acid at a temperature of from about 160 to about 212 F. In a typical operation, the article, after cleaning by any of the methods described above, is treated with glycolic acid solution by either dipping or spraying.

The concentration of glycolic acid in an aqueous solution suitable for use in this process, varies from about 1% to about 20% by weight, preferably, the solution will contain approximately 1 to 4% by weight of gly- Patented Dec. 5, 19.61

erably from 190 to 212 F. Higher temperatures within this range will generally be employed if the solution is to be applied by spraying because of the temperature drop inherent in any spraying operation. The treatment is continued from about 5 to about 30 minutes depending upon the gauge of the metal. Since metals other than 12 to 28 gauge are rarely .porcelainized, this time range is usually adequate. With 20 gauge steel, the time will be from about 5 to about 20 minutes;

The article to be porcelainized is next placed in a nickel sulfate bath. The purpose of this treatment is to deposit a coating of nickel on the surface of the metal. This deposit enhances the strength of the bond between the metal and the poreclain enamel. Typically, the nickel sulfate will be prepared in a concentration of approximately /2 ounce to 3 ounces per gallon of water in a wooden tank or a tank lined with lead or brick. The metal is immersed in the nickel sulfate solution for a period of from about 4 to about 8 minutes at a temperature of from about 160 to about 170 F. For optimum results, the pH of the solution is maintained between 3.2 and 3.6, preferably 2.8 to 3.2 by the addition of acid, preferably mineral acids such as sulfuric although at the higher pH, organic acids can be used. When a metal surface is treated in this manner, there is deposited a surface coating of nickel comprising from about 0.03 to about 0.1 gram per square foot. The article is then rinsed with water, preferably by immersion in a water bath at from about 55 to aboutr65 F.

. Following this rinse, the metal is Washed with a neutralizing solution, although other solutions may occasionally be used. The preferred solution contains borax and soda ash in a ratio of approximately 2:1. The mixture is dissolved in water at a concentration of approximately /2 ounce per gallon. The temperature is maintained at from about 165 to about 212 F. during the neutralizing operation. The time is from about 2 to about 15 minutes. After the neutralizing operation, the article is rinsed with water at from about 55 to about 212 F. and dried preferably at from about 185 to about 212 F.

The article is now ready for the application of the enamel slip, that is, the porcelain coating. The coating may be any ofthe various enamel coatings available commercially for cover coats containing the usual refractory materials, opacifiers, tiocculating agents, etc. It may also contain coloring agents. The enamel is applied by.

spraying or dipping in accordance with standard industrial practice. The article is then dried at from about 250 to about 400 'F. and baked at from about 1300 to about 1550 F. for from about 3 to about 6 minutes; In preferred operations, the article is baked at from about 1350 to about 1500 for a period of from about 2%. to about minutes.

In the practice of the improved process of this invention, sufficient alkali metal thiosulfate or sulfite is present in the aqueous glycolic acid pickle bath described above so as to maintain a concentration of from about 0.0012 to 0.0030 mole per gallon of alkali metal thiosulfate or 0.0018to 0.0042 mole of alkali metal sulfite. In preferred operations, the concentration is maintained at from about 0.0018 to about 0.0030 mole per gallon in alkali metal thiosulfate or 0.0030 to 0.0042 mole per gallon in sodium sulfite.

All calculations are made based on the anhydrous form with sodium thiosulfate pentahydrate the desired concentrations in grams are from 0.2 to 0.5 gram per gallon. With sodium sulfite they are 0.3 to 0.7 gram per gallon. Other concentrations are readily calculated from the known molecular weights or the salts.

The desired initial concentration of added reagent is readily obtained by weighing out the appropriate quantity of the selected compound and adding it to the bath. The concentration decreases as the bath is utilized in the pickling operation. The desired concentration of sodium thiosulfate can be maintained by the addition of 0.1 gram per gallon of this reagent in the pentahydrate form after each 3 square feet of ferrous metal per gallon hasbeen pickled, for example, with a 500 gallon pickle bath, an

additional grams should be added after 1500 square feet of ferrous metal has been pickled. With sodium sulfite, an additional 0.2 gram per square foot is added after each 3 square feet of ferrous metal per gallon has been treated; Alternatively, aliquots of the bath can be withdrawn and analyzed by known chemical means and the exact quantity required to maintain the desired concentration can be determined and added- Equivalent quantities of the other alkali metal salts can be readily calculated and employed.

Colloidal sulfur is produced when sodium thiosulfate is employed but it is apparent that this does not effect the desired result since no colloidal sulfur is produced when sodium sulfite is employed.

The improvement of this invention, even though the balance of the process is carried out in exact conformance with the earlier procedure, unexpectedly decreases the pickling period to from about 3 to about 15 minutes without sacrifice in appearance or bond in the final product.

It is specifically intended to include within the purview of this invention the pickle bath itself comprising an aqueous solution containing from about 5% to about 20% by weight of citric acid and from about 0.0012 to about 0.0030 mole per gallon of alkali metal thiosulfate or from about 0.0018 to about 0.0042 mole per gallon of alkali metal sulfite.

The following examples are given solely for the purpose of illustration and are not to be construed as limitations of this invention, many apparent variations of which are possible without departing from the spirit or scope thereof.

Example I A two-foot by twelve-foot refrigerator baffle fabricated from an 18 gauge plate of a highly purified steel having a carbon content of 0.003% was cleaned by immersion in an alkaline cleaner and rinsed according to standard practice. It was then immersed in a 5% by weight aqueous solution of glycolic acid containing 0.0018 mole of sodium thiosulfate per gallon and maintained at a temperature of 190 F. for 10 minutes. It was removed and rinsed with water at room temperature. The baflle was then immersed in a nickel sulfate aqueous solution containing nickel sulfate at a concentration of '1 ounce per 'gallon for a period of 4 minutes while maintaining the pH between 3.0 and 3.2 by the occasional addition of sulfuric acid. The temperature was maintained at 160 F. This treatment deposited approximately 0.12 gram of nickel per square foot of steel. The bafiie was then rinsed in water at room temperature and immersed in an aqueous solution containing soda ash and borax in a ratio of 2:1 at a concentration of /2 ounce per gallon. The temperature was maintained at F. for 3 minutes. The baflie was then dried by hot air, coated by spraying with commercial porcelain enamel and dried at about 250 F. The battle was then baked at 1400 F. for 4 minutes. The bond and appearance were excellent.

It was found that in order to obtain comparable bond and appearance under identical conditions except that the glycolic acid bath did not contain added sodiumthiosulfate, it was necessary to maintain the baffle in the pickle bath for 20 minutes.

Similar results are obtained with the thiosulfate salts of potassium lithium and cesium.

Example 11 was then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of two ounces per gallon for a period of 8 minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of solid boric acid to deposit on 26 grams of nickel per square foot of surface. The temperature was maintained between 160 and 170 F. The article was then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 at a concentration of one-half ounce per gallon. The temperature was maintained at approximately 165 F. for 15 minutes. The article was then rinsed with water at 185 F. and dried. It was coated by immersion with commercial porcelain enamel, dried at 400 F. and baked at 1450 F. for 3 minutes. The bond and appearance was excellent.

It was found that in order to obtain comparable bond and appearance under identical conditions except that the glycolic acid bath did not contain added sodium thiosulfate, it was necessary to maintain the panel in the pickel bath for minutes.

Similar results are obtained with the thiosulfates of potassium, lithium and cesium.

Example III A 3 inch by 6 inch piece of 12 gauge enameling iron containing 0.0003% carbon was cleaned by immersion in aqueous sodium hydroxide and rinsed according to standard practice. It was then immersed in a 4% by weight aqueous solution of glycolic acid containing 0.0030 mole of sodium thiosulfate per gallon at 160 F. for minutes. It was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of 2 ounces per gallon for a period of 4 minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of solid glycolic acid. The temperature was maintained between 160 and 170 F. This treatment deposited approximately 0.36 gram of nickel per square foot of surface. The piece was then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 at a concentration of one-half ounce per gallon. The temperature was maintained at approximately 212 F. for 15 minutes. The article was then rinsed with water at 212 F. and dried. It was coated with commercial porcelain enamel, dried at about 250 F., and baked at 1350 F. for 4 minutes. The appearance and bond was of commercial grade.

B was found that in order to obtain comparable bond and appearance under identical conditions except that the glycolic acid bath did not contain sodium thiosulfate, it was necessary to maintain the article in the pickel bath for 30 minutes.

Similar results are obtained using potassium, lithium and cesium.

Example IV A 3 in. by 6 in. piece of 16 gauge mill steel was cleaned with an emulsion type cleaner followed by immersion in 30% aqueous sodium hydroxide. It was then immersed in a 15% by weight aqueous solution of glycolic acid containing 0.0018 mole of sodium sulfite per gallon at 212 F. for 3 minutes. It was removed and rinsed with water at room temperature. The article was then immersed in a nickel sulfate bath containing nickel sulfate at a concentration of 4 ounces per gallon for a period of 4 minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of solid citric acid. The temperature was maintained between 160 and 170 F. The article was then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 at a concentration of /2 ounce per gallon. The temperature was maintained at approximately 212 F. for 2 minutes. The article was then rinsed with water at room temperature and dried. It was coated by spraying with commercial porcelain enamel, dried at about 350 F. and baked at 1450 F. for

4 minutes. The enamel coating was of commercial quality. 7

It was, found that in order to obtain comparable bond and appearance under identical conditions, except that the glycolic acid bath did not contain added sodium sulfite, it was necessary to maintain the article in the pickle bath for 6 minutes.

Similar results were obtained with the sulfite salts of potassium, lithium and cesium.

Example V A 3 in. by 6 in. piece of 16 gauge steel containing 0.03% carbon was cleaned with an emulsion type cleaner followed :by immersion in aqueous sodium hydroxide. It was then immersed in a 10% by weight aqueous solution glycolic acid containing 0.0042 mole of sodium sulfite per gallon at 180 F. for 3 minutes. It was removed and rinsed with water at room temperature. The article was then immersed ina nickel sulfate bath containing nickel sulfate at a concentration of 4 ounces per gallon for a period of 4 minutes while maintaining the pH between 3.2 and 3.6 by the occasional addition of sulfuric acid. The temperature was maintained between and F. The article was then immersed in an aqueous solution containing borax and soda ash in a ratio of 2:1 a concentration of /2 ounce per gallon. The temperature was maintained at approximately 212 F. for 2 minutes. The article was then rinsed with water and F. and dried. It was coated by spraying with commercial porcelain enamel, dried at about 350 F. and baked at 1450 F. for 4 minutes. The bond and appearance were of commercial quality.

It was found that in order to obtain comparable bond and appearance under identical conditions, except that the glycolic acid bath did not contain added sodium sulfite, it was necessary to maintain the article in the pickle bath for 6 minutes.

Similar results were obtained with the sulfite salts of lithium, potassium and cesium.

What is claimed is:

1. A process which comprises pickling clean ferrous metal objects with an aqueous solution of glycolic acid containing from about 1% to about 20% by weight of acid together with a reagent selected from the group consisting of alkali metal thiosulfates and alkali metal sulfites the concentration of alkali metal thiosulfate being from about 0.0012 to about 0.0030 mole per gallon and the concentration of alkali metal sulfite being from about 0.0018 to about 0.0042 mole per gallon at a temperature of from about 160 to about 212 F. for from about 3 to about 15 minutes, depositing on the surface thereof from about 0.12 to about 0.36 gm. of nickel per sq. ft., applying a procelain enamel coating, drying at from about 250 to about 400 F. and baking at from about 1350 F. to about -1450 F. for from about 2 to about 6 minutes.

2. Process as in claim 1 wherein the alkali metal thiosulfate is sodium thiosulfate.

3. A process as in claim 1 wherein the alkali metal sulfite is sodium sulfite.

4. In a process for the porcelain enameling of ferrous metal objects comprising the steps of cleaning the ferrous metal, pickling with an aqueous solution of glycolic acid containing from about 1% to about 20% by weight of acid at a temperature of from about 160 to about 212 F., depositing nickel on the surface in an amount of from about 0.12 to about 0.36 gm. per sq. ft. treating the object with nickel deposited on the surface thereof with porcelain enamel, drying at from about 250 to about 400 F. and baking at from about 1350 to about 1450 'F. for from about 2 to about 6 minutes, the improvement which comprises adding a sufficient amount of a reagent selected from the group consisting of alkali metal thiosulfates and alkali metal sulfites to maintain a concentration of from about 0.0012 to about 0.0030 mole per gallon in the case of alkali metal thiosulfates and from about 0.0018 to about 7 0.0042 mole per gallon in the case of alkali metal sulfites and pickling for from about 3 to about 15 minutes.

5. A pickling bath comprising an aqueous solution containing from about 1% to about 20% by weight of glycolic acid together with a reagent selected from the group con- 5 sisting of alkali metal thiosulfates and alkali metal sulfites, the concentration of alkali metal salts being from about 0.0012 to about 0.0030 mole per gallon in the case of alkali metal thiosulfates and from about 0.0018 to about 0.0042 mole per gallon in the case of alkali metal sulfites. 10

References Cited in the file of this patent UNITED STATES PATENTS 7 Chester Oct. 30, 1956 

1. A PROCESS WHICH COMPRISES PICKLING CLEAN FERROUS METAL OBJECTS WIT AN AQUEOUS SOLUTION OF GLYCOLIC ACID CONTAINING FROM ABOUT 1% TO ABOUT 20% BY WEIGHT OF SAID TOGETHER WITH A REAGENT SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL THIOSULFATES AND ALKALI METAL SULFITES THE CONCENTRATION OF ALKALI METAL THIOSULFATE BEING FROM ABOUT 0.0012 TO ABOUT 0.0030 MOLE PER GALLON AND THE CONCENTRATION OF ALKALI METAL SULFITE BEING FROM ABOUT 0.0018 TO ABOUT 0.0042 MOLE PER GALLON AT A TEMPERATURE OF FROM ABOUT 160* TO ABOUT 212*F. FOR FROM ABOUT 3 TO ABOUT 15 MINUTES, DEPOSITING ON THE SURFACE THEREOF FROM ABOUT 0.12 TO ABOUT 0.36 GM. OF NICKEL PER SQ. FT APPLYING A PROCELAIN ENAMEL COATING, DRYING AT FROM ABOUT 250* TO ABOUT 400*F. AND BAKING AT FROM ABOUT 1350* F. TO ABOUT 1450*F. FOR FROM ABOUT 2 TO ABOUT 6 MINUTES. 